The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Low pressure radio frequency (RF) generated plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created within the plasma impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 1% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
In order to maximize the energy being coupled from the coil to the plasma, it is desirable to position the coil, as close as possible to the plasma itself. At the same time, however, it is also desirable to minimize the generation of particles being shed from interior surfaces. These particles shed from interior surfaces can fall on the wafer itself and contaminate the product. Accordingly, many sputtering chambers have a generally annular-shaped shield enclosing the plasma generation area between the target and the pedestal supporting the wafer. The shield provides a smooth gently curved surface which is relatively easy to clean and protects the interior of the vacuum chamber from being deposited with the sputtering material. In contrast, it is believed that a coil and any supporting structure for the coil generally have relatively sharply curved surfaces from which it would be more difficult to clean away materials deposited from the coil and its supporting structures. In addition, it is believed that the smooth gently curved surface of the shield would tend to shed fewer particles than the sharply curved surfaces of the coil and its supporting structure.
Thus, on the one hand, it would be desirable to place the coil outside the shield (as described in application Ser. No. 08/559,345, filed Nov. 15, 1995 now U.S. Pat. No. 6,264,812 for METHOD AND APPARATUS FOR LAUNCHING A HELICON WAVE IN A PLASMA which is assigned to the assignee of the present application and is incorporated herein by reference) so that the coil is shielded from the material being deposited. Such an arrangement would minimize generation of particles by the coil and its supporting structure and would facilitate cleaning of the chamber. On the other hand, it is desirable to place the coil as close as possible to the plasma generation area inside the shield, to avoid any attenuation by the spacing from the plasma or by the shield itself, and thereby to maximize energy transfer from the coil to the plasma. Accordingly, it has been difficult to increase energy transfer from the coil to the plasma while at the same time minimizing particle generation and facilitating chamber cleaning.
It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter depositing a layer which obviate, for practical purposes, the above-mentioned limitations.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention a plasma generating apparatus which inductively couples electromagnetic energy from a coil which is magnetically shielded from the plasma and ionized target material so as to minimize the deposition of target material onto the coil. As a consequence, contamination of the workpiece by particulate matter shed by the coil may be reduced.
In a preferred embodiment, an apparatus for energizing a plasma within a semiconductor fabrication system to sputter material onto a workpiece may include a semiconductor fabrication chamber having a plasma generation area within the chamber, a coil carried by the chamber and positioned to couple energy into the plasma generation area, and an arrangement of magnetic fields positioned to reduce sputtering from the coil onto the workpiece. Alternatively, the arrangement of magnetic fields may by positioned also to reduce sputtering from the target onto the coil. The target and coil may both include titanium, aluminum or other suitable materials. The apparatus may include one or more electromagnet coils carried by the chamber and positioned to provide the arrangement of magnetic fields. Alternatively, the apparatus may include one or more permanent magnets carried by the chamber and positioned to provide the arrangement of magnetic fields.